ML20239A143
| ML20239A143 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 08/26/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20239A142 | List: |
| References | |
| NUDOCS 9809080260 | |
| Download: ML20239A143 (7) | |
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arg j-h UNITED STATES I
NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. enman -g
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.140 TO FACILITY OPERATING LICENSE NO. NPF-10 AND AMENDMENT NO.132 TO FACILITY OPERATING LICENSE NO. NEE-15 SOUTHERN CALIFORNIA EDISON COMPANY SAN DIEGO GAS AND ELECTRIC COMPANY THE CITY OF RIVERSIDE. CALIFORNIA THE CITY OF ANAHEIM. CALIFORNlt SAN ONOFRE NUCLEAR GENERATING STATION. UNITS 2 AND 3 DOCKET NOS. 50-361 AND 50-362
1.0 INTRODUCTION
By application dated September 16,1997, as supplemented by letter dated February 23,1998, Southem Califomia Edison (the 2censee), submitted a request to change the Technical Specifications (TS) at San Onofre Nuclear Generating Station (SONGS), Units 2 and 3. The proposed changes would allow sleeving of steam generator tubes with sleeves designed by the vendor, ASEA Brown Boveri/ Combustion Engineering (ABB/CE). Additionally, the proposed TS amendment would require that sleeves be removed from service upon detection of service-induced degradation, require post weld heat treatment.(PWHT) of sleeve welds, and reduce the allowable primary to secondary leakage through any one steam generator to 150 gallons per day (gpd).-
The revised TS would reference the current generic topical report for ABB/CE welded sleeves, CEN-630-P, Revision 2," Repair of 3/4 0.D. Steam Generator Tubes Using Leak Tight Sleeves," dated June 1997, and 96-OSW-003-P, Rev. 00,"EPRI Steam Generator Examination Guidelines - Appendix H Qualification For Eddy Current Plus-Point Probe Examination of
. ABB/CE Welded Sleeves." Because the bulk of the technical and regulatory issues for the present request are identical to those reviewed in the previous Safety Evaluations (SEs) for ABB/CE sleeves, this SE discusses only those issues arranting revision, amplification or n
inclusion based on current experience.
- Details of prior staff evaluations of ABB/CE sleeves may also be found in the SEs for Waterford Steam Electric Station, Unit 3, Docket No. 50-382, dated December 14,1995; Byron Nuciaar Power Station, Units 1 and 2 and Braidwood Nuclear Power Station, Units 1 arid 2, Docket Nos.
50s454,50-455,50-456, and 50-457, dated April 12,1996; Zion Nuclear Power Station, Units 1 P
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and 2, Docket Nos. 50-295 and 50-304, dated October 29,1996; Prairie Island, Units 1 and 2, Docket Nos. 50-282 and 50-306, dated November 4,1997; and Beaver Valley, Unit 1, Docket No. 50-334, dated November 25,1997. These evaluations apply to the proposed SONGS 2 and 3 license amendments.
The February 23,1998, supplemental letter provided additional clarifying information and did not change the staff's original no significant hazards consideration determination published in the Federal Register on January 28,1998 (63 FR 4323).
2.0 BACKGROUND
Previous staff evaluations of ABB/CE sleeves addressed the technical adequacy of the sleeves in the four principal areas of pressure retaining component design: structural requirements, material of construction, welding, and non-destructive examination. The staff found the analyses and tests that were submitted by Southem Califomia Edison provided technical justification that are sufficiently bounded by these other licensees'submittals. The staff, therefore, found these areas of component design to be acceptable for SONGS Units 2 and 3.
The function of sleeves is to restore the structural and leakage integrity of the tube pressure boundary. Consequently, structural analyses were performed for a variety ofloadings including design pressure, operating transients, and other parameters selected to envelope loads imposed during normal operating, upset, and accident conditions. Stress analyses of sleeved tube assemblies were performed in accordance with the requirements of the ASME Boiler and Pressure Vessel Code (the Code), Section Ill. The licensee cited these analyses, along with the results of qualification testing and previous plant operating experience, to demonstrate that the sleeved tube assembly is capable of restoring steam generator tube integrity.
The two proposed ABB/CE sleeve types are an expansion transition zone (ETZ) sleeve and a tube support plate (TSP) sleeve. An ETZ sleeve is designed to restore the portion of a tube in the vicinity of the top of the steam generator (SG) tub 6 sheet. A TSP sleeve can be used to span a support plate elevation or be used on a freespan section of tube. The sleeve materialis a nickel-iron-chromium alloy, Alloy 690, a Code approved material (ASME SB 163),
incorporated in ASME Code Case N-20.
The ABB/CE sleeves are installed using gas tungsten arc welding to join the sleeve to the parent tube at the upper (free span) end of the ETZ sleeve and at both ends of a TSP sleeve.
The lower ETZ sleeve tube joint is hard-rol!ed into the tubesheet below the expansion zone.
The centerline of the welds forms the pressure boundary transition behveen the sleeve and the tube. Considering the unreliability of sleeve life predictions, the staff has typtally required l
licensees to inspect a sampio of s'seves at eabh outtge. Periodic inservloe inspections will provide assurance that any service induced degradation in sleeves is detected and addressed appropriately. Inservice inspection requirements applicable to the licensee's proposed amendment request are discussed further in Section 3.4 of this safety evaluation.
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,M' 3.0 - DISCUSSION Experience with all types of SG tube sleeves has led to several areas of concem outside the scope of basic sleeve design and qualification discussed in the previous SEs. These include weld preparation, weld acceptance inspections, service life predictions for sleeved SG tubes, sleeve plugging limits, post weld heat treatment, and primary-to-secondary leakage limits.
3.1.
Wald Preparation Prior to performing any weld, the surface of the metal (s) to be welded must be cleaned. For sleeve installation, the inner diameter of the parent tube at the desired weld location must be cleaned of service induced oxides. For the ABB/CE sleeving process, this is accomplished
- using motorized wire brushes.
Based on recent findings during the inspection of sleeves at other plants, ABB/CE revised the
- cleaning method to ensure optimum removal cf service induced oxides. The revised cleaning procedure entailed some equipment changes. More significantly, from a quality assurance standpoint, a 100 percent visual testing (VT) of the cleaning process was instituted. After the wire brush cleaning step, every tube is given a VT using a remote fiber optic camera system to confirm that adequate surface cleaning has been accornplished. As required by proposed TS 5.5.2.11.f.1(l), the weld preparation VT inspection program requires a full length inspection of previously plugged tubes that are sleeved prior to retuming the tube to service. This provides for tubes that had previously been plugged, and thus out of service for some time period, to be fully inspected for potential defects in other than t'ne area that was sleeved prior to restoring the tube to service. This provides reasonable assurance that no additional defects exist in the tubes prior to retum to service, and is, therefore, acceptable.
3.2 Weld Acceptance inspections For compliance with the Code and regulatory requirements, initial and periodic examinations of steam generator tubes and sleeves are performed. Sleeve welds were Nptorically accepted based on VT and ultrasonic testing (UT) examinations. Eddy current testing (ET) was used for an initial baseline inspection for comparisor with later required periodic inspections. The reason for the different types of nondestructive examinations (NDE) being used for initial acceptance versus periodic reinspection is due to the differences between potential flaws from
' initialinstallation defects and service induced degradation. The differerst NCE techniques have normally been better suited for the respective types of anticipated flaws.
Past field experience syggested that the current initial acceptance examinations (VT and UT) may not be sufficient in every circumstance. As a result, the weld acceptance NDE was
~ modified to include:
L 100 percent UT with an enhanced digitized amplitude system L
100 percent ET using the Plus Point probe The original UT procedure was based upon the absence of a mid-wall reflection. In that procedure, the sleeve cuiside diameter wall reflection was reac'ily apparent beyrnd the fusion p
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zone of the weld, thus signifying lack of fusion with the parent tube. When fusion existed, the mid-wall reflection (mid-wall of the fused sleeve and tube combination) would not appear since l
no interface would exist. Field experience led ABB/CE to discover that lack of fusion caused by
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axially oriented oxide inclusion from a poorly cleaned weld would not be detected since the
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oxides did not cause a large sound reflection.
I in the enhanced UT procedure, the back wall signal from the outside of the parent tube is also monitored for presence in the fused area.' Additionally, the back wall signal strength is -
- examined for excessive attenuation. Attenuation beyond the normal amount can be interpreted, along with other signal artifacts, as either a weld that is too narrow or one with inclusions or patches of unfused material. The modified UT procedure was extensively tested on laboratory produced welds containing a variety of inclusion / lack of fusion defects. Samples were destructively examined and the metallurgical sections compared with the UT results.
Comparison of results demonstrated the revised UT procedure was highly reliable, and that no significant defects could remain undetected by the enhanced UT procedure.
ET with the plus point probe is now part of the sleeve weld acceptance criteria. Field experience led ABB/CE to discover that weld suckback and circumferentially oriented oxide inclusions from a poorly cleaned weld would not be detected by UT. ABB/CE has shown the plus point probe reliably detects the various process-induced weld defects including blowholes, weld suckback and circumferentially oriented oxide inclusions. ABB/CE has also shown the ET
. can reliably locate the position of the defect with respect to the weld centerline which is considered the pressure boundary. Any ET indication found below the weld centerline of the upper weld or above the weld centerline of the lower weld (i.e., inside the sleeve pressure boundary) requires the tube to be plugged if the ET indication does not meet the UT requirements.
The proposed inspections provide reasonable assurance that sleeved tuses will be placed in service only after verification of acceptable installation. The staff, therefore, finds the proposed inspections acceptable.
3.3 inservice inspection Requirements included in the licensee's proposed amendment request are changes that would require the licensee to perform an inspection of a number of sleeves at each refueling outage. The minimum sample requirements for tube inspections, which are specified in " Steam Generator Sample Selection and inspection" within TS 5.5.2.11, are established to assess the overall condition of the steam generator. Because sleeved tubes are of a slightly different configuration and may be more susceptible to stress corrosion cracking than unrepaired tubing,
- the inservice inspection requirements currently specified in the TS may not be sufficient to address the condition of these tubes.
The licensee has proposed to include additional inservice inspection requirements in the TS to address sleeves. The changes would require the inspection of at :6ast 20 percent of all installed sleeves. This proposal is consistent with current industry guidance for steam generator sfeeve examinations. EPRI recommends a 20 percent sample inspection for sleeves.
- In addition to the licensee's proposal, the results from inspections would be classifed and, i
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C depending on the classification, may require the performance of additional inspections of sleeves.
l The proposed inspection requirements provide reasonable assurance that service-induced l
degradation in SG tubes sleeves will be detected thus minimizing potential events related to SG l
tube integrity and are, therefore, acceptable.
3.4 Sleeve Plugging Limits The sleeve minimurn ecceptable wall thickness is determined using the criteria of Regulatory Guide 1.121," Bases for Plugging Degraded PWR Steam Generator Tubes" and ASME Code Section ill allowable stress values and pressure stress equations. The sleeve structurallimit, which was calculated based on the most limiting of normal, upset, or faulted conditions for 3/4-inch outside diameter steam generator tubes in ABB/CE designed generators, was I
determined to be 4g percent of the sleeve nominal wall thickness based on ASME Code minimum material properties in accordance with staff positions. However, the licensee has proposed to remove sleeves from service upon detection of service-induced degradation of the sleeve material or any portion of the sleeve-to-tube weld which provides assurance that twes will maintain adequate margins for structural integrity during the next cycle of operation.
The proposed change is more conservative that industry and staff acceptance criteria, and provides reasonable assurance that tube structural integrity is maintained throughout each l
operating cycle and is, therefore, acceptable.
3.5 Post Weld Heat Treatment (PWHT)
Accelerated corrosion tests confirm that a PWHT significantly improves the intergranular stress corrosion cracking (IGSCC) resistance of the alloy 600 parent tube material in the weld zone.
Proposed TS 5.5.2.11.f.1.(j) requires a PWHT of the welded joints in accordance with the ABB/CE topical report, CEN-630-P, Revision 2. The staff has determined that this topical report is appropriate for use at SONGS Units 2 and 3. Since this treatment improves resistance to IGSCC, the staff finds the proposed change acceptable, l
3.6 Primary-to-Secondary Leakage Limits The licensee proposes to adopt a 150 gallon per day (gpd) limit on primary-to-secondary leakage in SGs with tube sleeves installed. This limit is more restrictive than the current TS limit of 720 gpd in any one SG, and would require earlier action to in the event of a tube leak to assure defense-in-depth for postulated accidents. Since the proposed change is more restrictive, the plant design remains bounded by the current SG tube rupture analysis and is, therefore, acceptable.
3.7 Change in Reporting Requirements The licensee is incorporating a change to TS 5.7.2 to modify the SG inspection reporting requirements to add reporting on the number and extent of sleeves that are inspected. This is an administrative change only that adds reporting requirements, and is therefore acceptable.
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c 3.8 Technical Specification Changes l
In order to incorporate the proposed changes to permit sleeving of the degraded tubes in San Onofre Nuclear Generating Station, Units 2 and 3, steam generators, the licensee has proposed the following changes to the TS. The evaluation of these changes is provided in the preceding sections of this SE.
- 1. Proposed Changes to TS 3.4.13 The allowable primary to secondary leakage through any one SG is reduced to 150 gallons per day.
- 2. Proposed Changes to TS 5.5.2.11.b," Steam Generator Tube Sample Selection and inspection" A new requirement is added to specify that a minimum sample inspection of 20 percent of the total number of sleeves in service is required. in addition, the results of the sleeve inspections are to be classified and additional actions taken, if necessary, per Table TS 5.5.2.11-2.
- 3. Proposed Changes to TS 5.5.2.11(f)," Repair Limit" The definition of Repair Limit is modified to specify that the sleeve shall be removed from service upon detection of service-induced degradation of the sleeve material or any portion of the sleeve-to-tube weld.
- 4. Proposed New TS 5.5.2.11(j)
A section is added to specify that tube repair using ABB/CE welded sleeves shall be in accordance with the methods described in ABB/CE topical reports CEN-630-P, Revision
- 2. In addition, the proposed section indicates that a PWHT of the sleeve welds as described in ABB/CE report CEN-630-P shall be performed.
- 5. Proposed New Table TS 5.5.2.11-2," Steam Generator Sleeved Tube Inspection" Table TS 5.5.2.11-2 is added to the TS to specify that a 20-percent sample inspection is required for tube sleeves. In addition, the table also states the actions (i.e., additional inspections) necessary based on the results classification from the initial sample.
l The staff has reviewed the proposed changes to the TS, and determined that sleeved tubes will continue to maintain safety margins consistent with the stress limits in ASME Section 111.
In addition, the proposed TS provide an acceptable basis to assure an adequate inspection program and operational leakage limits to assure the plant is operated within its current design basis for SG tube integrity. The staff, therefore, finds the proposed changes acceptable.
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4.0 STATE CONSULTATION
'In accordance with the Commission's regulations, the Califomia State official was notified of the l
L proposed issuance of the amendmeits. The State official had no comments.
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5.0 ENVIRONMENTAL CONSIDERATION
The amendments change a requirement with respect to the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and surveillance requirements. The NRC staff has determined that the amendments involve no significant incmase in the amounts, and no significant change in the types, of any effluents that may be L
released offsite, and fW. there is no significant increase in individual or cumulative occupational j
radiation exposure. %e Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (63 FR 4323). The amendments also change recordkeeping or reporting requirements. Accordingly, the amendments rneet the eligibility criteria for categorical 1
exclusion set forth in 10 CFR 51.22(c)(9) and (c)(10). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.
6.0 CONCLUSION
l The Commission has concluded, based on the considerations discussed above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by l.
operation in the proposed manner, (2) such activities will be conducted in compliance with the l
Commission's regulations, and (3) the issuance of the amendments will not be inimical to the,
I common defense and security or to the health and safety of the public.
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PrincipalContributor: E. Debec-Mathet l:
l-Date:
August 26, 1998 L
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